This invention relates to exhaust nozzles used with turbofan engines, and more particularly to an exhaust nozzle used with a turbofan jet engine for reducing the noise of the exhaust gasses emitted from the engine without suppressing the flow of exhaust gasses through the exhaust nozzle.
The reduction of exhaust flow jet noise from turbofan aircraft engines is essential to meeting current and anticipated future government regulatory requirements for Airplane Type Certification, as well as numerous local airport noise ordinances. There have been many attempts to accomplish jet exhaust noise reduction through various modifications to the exhaust nozzle of the engine. While many of these attempts have produced some degree of noise reduction, they have also resulted in adverse impacts on engine operability and/or engine operability limit related performance.
A jet noise reducing nozzle segment is typically triangular in planform and is contoured to increasingly immerse or extend into the exhaust flow with distance along its length. Multiple segments attached to the exit of a conventional exhaust nozzle are typically used to form a jet noise reducing segmented exhaust nozzle. The effective flow area of the exhaust nozzle is reduced when nozzle segments are employed due to the presence of portions of the nozzle segment projecting into the exhaust gas flow path. These portions, in effect, present additional blockage to the oncoming exhaust gas flow. On a turbofan engine, the additional blockage results in reduced fan flutter margin which can negatively impact fan aero-elastic structural stability. It also can cause increased exhaust gas temperatures which can negatively impact turbine life. Still further, it can result in reduced engine compressor stall margin which can negatively impact engine core operation stability. Individually or together, these impacts can be of such significance that they prevent the implementation of the noise-reducing device on a jet engine. This impact is particularly hard felt on older jet engine designs that have been xe2x80x9cthrust bumpedxe2x80x9d to near their operational limits.
With ever increasing stringency of new community noise limitations, existing aircraft types currently in service, as well as new designs for future aircraft, will require new jet noise control technology. This increased stringency could potentially present a threat to the introduction of future aircraft designs. Moreover, ever stricter community noise limitations, if not addressed by suitable noise reduction technology, could impede the introduction of derivative aircraft platforms.
Accordingly, there exists a need to further reduce the noise produced by turbofan jet aircraft engines without imposing an unacceptable reduction in engine operability margins or operability limit related performance.
The present invention is directed to a segmented exhaust nozzle that effectively reduces the exhaust jet noise generated by a turbofan jet aircraft engine without adversely impacting engine operability or operability limit related performance. The exhaust nozzle is formed by a fan inner wall and a fan outer wall. The inner and outer walls cooperatively form an annular exhaust gas flow path therebetween. The walls further define a nozzle throat area and a nozzle exit area from which the exhaust gasses of a turbofan engine associated with the exhaust nozzle are emitted.
The exhaust nozzle of the present invention provides a first region in which one of the inner or outer walls curves gradually towards the other, thereby presenting reduced cross-sectional area to exhaust gas flow in this region forming an aerodynamic throat. A second region, through the segmented portion of the nozzle and downstream of the first region, is formed by the one wall curving away from the other wall to produce a region of increased cross-sectional area to exhaust gas flow. Importantly, the second region forms an exhaust gas nozzle exit area which has an effective cross sectional area approximately equal to a conventional exhaust gas nozzle exit area. This is in contrast to previously developed, segmented exhaust nozzles in which the exhaust gas nozzle exit area is smaller in cross section than a conventional exhaust nozzle exit area. This difference effectively serves to eliminate the negative impact on engine operability and operability limit related performance introduced by previous segmented exhaust nozzle configurations while still providing a significant reduction in engine jet noise.
The present invention thus reduces significantly the exhaust gas flow suppression that would typically be present with previous forms of segmented exhaust nozzles by presenting a geometric inflection through the flow control region of the nozzle. The inflective profile creates a conversion-divergent, cross sectional shape to the nozzle wall. The result is a net zero change in exhaust flow characteristic and a net zero change in segmented nozzle noise suppression effectiveness.